>80% of the then-current local rental rates for similar spaces.
That is to say, you benchmark it. If someone across the street is renting their place for $1000, and the city says you cant rent yours, they would have to pay you $800 to keep it empty.
Benchmarking sounds complicated, but is extremely common. My city reassesses my property value using benchmarks every year to determine property taxes. Of course, they use totally unrealistic numbers, and I have to call them out on it, and then they back down because they are indefensible.